In the related art, as an optical scanning device installed in an electrophotographic image forming apparatus, there has been known an optical scanning device including a light source, a polygon mirror unit that allows light beams emitted from the light source to be scanned in a main scanning direction, an image forming lens that forms an image of the light beams scanned by the polygon mirror unit on a surface to be scanned, and a housing that receives these optical devices therein (see Patent literature 1). The housing has a bottomed box shape opened upward and an upper side of the housing is closed by a lid member.